The present invention relates to a soft-nitrided steel part having high fatigue strength, excellent bending properties and excellent wear resistance, even if subjecting to soft-nitriding without quenching-tempering or normalizing after hot working.
Generally, shaft parts for automobiles, industrial machinery, construction machinery and the like are produced by the following process, in order to achieve their desired strength.
After forming billets of alloy steels or carbon steels for machine structural use into a desired shape through hot forging, machining and the like, the shaped workpiece is subjected to (a) a heat treatment (e.g. induction quenching-and-tempering), or (b) further subjected to soft-nitriding after the said heat treatment. The soft-nitriding can further improve fatigue strength of the steel parts after heat treatment.
In late years, for cost saving, various processes, i.e., so-called xe2x80x9cnon-heat treatment soft-nitriding processxe2x80x9d or xe2x80x9cnon-normalizing soft-nitriding processxe2x80x9d, which intend to eliminate quenching-tempering or normalizing before soft-nitriding, have been discussed. However, when alloy steels or carbon steels for machine structural use defined by JIS (Japanese Industrial Standards) are used for materials, eliminating quenching-tempering or normalizing before soft-nitriding allows a coarsened grain structure or a mixed grain structure during hot working to be retained in a final product. These structures deteriorate fatigue strength and bending properties of the final product. In high-speed rotating shafts, such as a crankshaft, wear at sliding portions thereof can give rise to oil leak or sticking.
Some approaches have been suggested to steel parts manufactured without quenching-tempering or normalizing before soft-nitriding, and manufacturing methods thereof, as described below.
(1) A steel for nitriding, including C, Si, Mn, P, S, Cr, sol. Al, Ti, Ca, Pb and N, and being capable of having performances equal or superior to a heat treated steel in fatigue strength and crack length risen out of bending correction, even if subjecting to nitriding without heat treatment, and a manufacturing method thereof (cf. Publication of Unexamined Japanese Patent Application No. 9-3601).
(2) A steel for nitriding, including C, Si, Mn, P, S, sol. Al, Ti, Ca, Pb and N, and being capable of having performances equal or superior to a heat treated steel in notch fatigue strength and crack length risen out of bending correction, even if controlling to reduce each content of Cr and V in impurities and subjecting to nitriding without heat treatment, and a manufacturing method for a nitrided steel product using said steel for nitriding as a raw material (cf. Publication of Unexamined Japanese Patent Application No. 10-46287).
(3) A crankshaft including C, Si, Mn, Ti, Al, N, S and Ca as well as optionally Pb, and being produced with controlling to reduce each content of P and V in impurities and subjecting to nitriding without heat treatment, and a manufacturing method thereof (cf. Publication of Unexamined Japanese Patent Application No.11-62943).
(4) A nitriding steel having a substantial ferrite-pearlite structure, 30% or more of ferrite area ratio, a grain size defined by five or over of ferrite grain size number, and 50 xcexcm or less of average size of pearlite, in the state just after subjecting a steel defined by C, Si, Mn, Cr, Ni, Mo, N, V, Nb, Ti, Zr, Ta, S, Pb, Ca, Bi and Te to hot working (cf. Publication of Unexamined Japanese Patent Application No. 9-291339).
(5) A soft-nitridied workpiece having a ferrite-pearlite structure defined by 50 xcexcm or less of average size of ferrite crystal grain and 50 xcexcm or less of average size of pearlite crystal grain, in the state just after subjecting a steel defined by C, Si, Mn, Cr, Ni, Mo, N, V, Nb, Ti, Zr, Ta, S, Pb, Ca, Bi and Te to hot forging (cf. Publication of Unexamined Japanese Patent Application No.9-324258).
Properties of wear, fatigue, and bending, required by soft-nitrided parts have an antithetical relationship to each other. Thus, in the case of eliminating quenching-tempering or normalizing, it is generally difficult to maintain all of the properties at respective desired levels. However, in all inventions of steels for nitriding steel or nitrided parts described in the above (1) to (5), the variance of wear properties has not been considered.
It is an object of the present invention to provide a non-heat treated soft-nitrided steel part having high fatigue strength, excellent bending properties, and excellent wear resistance, even if it is subjected to soft-nitriding with neither quenching-tempering nor normalizing, i.e., without prior heat treatment after hot working (hereafter this process is referred to as xe2x80x9cnon-heat treatment soft-nitridingxe2x80x9d).
More specifically, it is an object of the present invention to provide a non-heat treated soft-nitrided steel part in which all of fatigue strength, bending properties and wear resistance are equal or superior to those of a steel part that is made of JIS-S48C (one of the machine structural use carbon steels) through soft-nitriding with subjecting to normalizing after hot working.
As a result of various studies for solving the foregoing problems, the present inventors have obtained the following knowledge.
(1) When a steel defined by a specific chemical composition is subjected to non-heat treatment soft-nitriding, the resulting wear resistance of the steel correlates with the following formula Fn1.
Fn1=xe2x88x92141.5(C %)xe2x88x9219.6(Mn %)+1280(N %)+95.6
In order to achieve the wear resistance equal or superior to that of a steel part for which a steel of JIS-S48C is selected as a raw material and which has been subjected to soft-nitriding after normalizing, 60 or less of Fn1 value is required.
(2) When a steel defined by a specific chemical composition is subjected to non-heat treatment soft-nitriding, the resulting fatigue strength of the steel correlates with the following formula Fn2.
Fn2=xe2x88x92103.8(C %)+59.1(Mn %)+850.4(N %)+360.9
In order to achieve the fatigue strength equal or superior to that of a steel part for which a steel of JIS-S48C is selected as a raw material and which has been subjected to soft-nitriding after normalizing, 350 or more of Fn2 value is required.
(3) When a steel defined by a specific chemical composition is subjected to non-heat treatment soft-nitriding, the resulting bending properties of the steel correlates with the following formula Fn3.
Fn3=xe2x88x9213.4(C %)xe2x88x923.45(Mn %)+112.7(N %)+13.2
In order to achieve the bending properties equal or superior to that of a steel part for which a steel of JIS-S48C is selected as a raw material and which has been subjected to soft-nitriding after normalizing, 7 or more of Fn3 value is required.
The present invention has been accomplished in accordance with the aforementioned knowledge, and a subject matter of the present invention is the following non-heat treated soft-nitrided steel part. A non-heat treated soft-nitrided steel part manufactured from a steel that contains, by mass %, 0.35 to 0.45% of C, 0.05 to 1.00% of Si, 0.3 to 1.0% of Mn, 0.03% or less of P, 0.15% or less of Cr, 0.001 to 0.03% of Ti, 0.03% or less of V, 0.010 to 0.020% of N, 0.08% or less of Al, 0.05 to 0.30% of Pb, 0.10% or less of S, and 0.003% or less of Ca, and satisfies the following formulas (1) to (3), with the remainder being Fe and incidental impurities, said steel part being subjected to soft-nitriding.
Fn1=xe2x88x92141.5(C %)xe2x88x9219.6(Mn %)+1280(N %)+95.6xe2x89xa660xe2x80x83xe2x80x83(1)
Fn2=xe2x88x92103.8(C %)+59.1(Mn %)+850.4(N %)+360.9xe2x89xa7350xe2x80x83xe2x80x83(2)
Fn3=xe2x88x9213.4(C %)xe2x88x923.45(Mn %)+112.7(N %)+13.2xe2x89xa77xe2x80x83xe2x80x83(3)